Increased legalization of marijuana for medical purposes may be fueling illicit use of the drug, as well as increasing the number marijuana use disorders, according to the results of a new study.

Researchers have identified a greater increase in illicit marijuana use and disorders in states that have legalized medical marijuana.

An analysis of data from three national surveys in the United States revealed a higher increase in illicit marijuana use and marijuana use disorders in states that had passed medical marijuana laws (MMLs), compared with states that have not legalized medical marijuana.

Study co-author Deborah Hasin, Ph.D., of the Mailman School of Public Health and the Department of Psychiatry at Columbia University Medical Center in New York, and colleagues say that their findings suggest that changing state marijuana laws may have serious consequences for public health.

In states that have passed MMLs, doctors may prescribe the drug to help treat a number of conditions, including pain, chemotherapy-related nausea, seizure disorders, and poor appetite and weight loss due to chronic illness.

Concerns about MMLs

While many people have welcomed the legalization of medical marijuana, others have raised concerns that it will increase recreational use of the drug, with some negative implications.

"Over time, the prevalence of adult illicit use and related consequences has increased. Thus, identifying factors underlying increased adult illicit use is important. State MML[s] may be one such factor."

For their study, Hasin and team investigated whether MMLs might be contributing to illicit use of the drug and marijuana use disorders.

The researchers analyzed data from 118,497 adults included in three national surveys: the 1991-1992 National Longitudinal Alcohol Epidemiologic Survey, the 2001-2002 National Epidemiologic Survey on Alcohol and Related Conditions, and the 2012-2013 National Epidemiologic Survey on Alcohol and Related Conditions-III.

These three surveys represent the changing scope of MMLs in the U.S. "In 1991-1992, no Americans lived in MML states; in 2001, 18.9 percent lived in MML states, and in 2012, more than one third (34.3 percent) lived in MML states," the authors note.

Greater increase in illicit marijuana use, disorders in MML states

The analysis revealed that between 1991-1992 and 2012-2013, states that had passed MMLs saw a greater increase in illicit marijuana use and marijuana use disorders than states that had not legalized medical marijuana.

The increase in illicit marijuana use and marijuana use disorders were 1.4 percent and 0.7 percent higher, respectively, in states that had legalized medical marijuana, compared with states that had no MMLs.

The biggest increases in illicit marijuana use were seen between 2001-2002 and 2012-2013. The rise in illicit use ranged from 3.5 percent in states without MMLs to 7 percent in states that had legalized medical marijuana.

Increases in rates of marijuana use disorders between 2001-2002 and 2012-2013 followed a similar pattern.

The team notes that further research is needed to establish what is driving the link between MMLs and increases in marijuana use and disorders in the U.S.

For now, the researchers believe that their findings demonstrate the potential negative implications associated with the legalization of medical marijuana.

"Medical marijuana laws may benefit some with medical problems. However, changing state laws - medical or recreational - may also have adverse public health consequences, including cannabis use disorders.

A prudent interpretation of our results is that professionals and the public should be educated on risks of cannabis use and benefits of treatment, and prevention/intervention services for cannabis disorders should be provided."

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